Optical pickup devices are well known to the art, which records and reproduces information by applying light beams to a recording medium such as an optical disk. In a commonly used optical pickup device, light beams projected from a semiconductor laser as a light source are passed through a diffracting element, and form a parallel pencil of light beams through a collimating lens. An intensity distribution of the parallel pencil of light beams shows an elliptical shape in its cross section orthogonal to the optical axis, according to the intensity distribution of the light beams projected from the semiconductor laser. In addition, such an intensity distribution of light related to a cross section orthogonal to the optical axis, is hereinafter referred to simply as the intensity distribution. The elliptical intensity distribution is shaped into a substantially circular shape by a shaping prism in order to improve the efficiency of light utilization in the optical pickup device. The light beams having their intensity distribution shaped into the substantially circular shape, are focused on the recording medium by an object lens.
The light beams reflected from the recording medium (hereinafter referred to simply as the reflected beams) follow a light path in a reversed manner to the above-mentioned, and have their intensity distribution restored to be elliptical by passing through the shaping prism. Thereafter, the reflected beams are directed to a diffracting element via the collimating lens, and diffracted by the diffracting element so as to be focused on a photodetector.
Normally, each of the diffracting element and the photodetector is divided into a plurality of regions by a plurality of division lines so as to obtain a focus error signal or a tracking error signal by adopting the knife-edge method or push-pull method. For example, in accordance with the knife-edge method, a focus error signal can be obtained by finding a difference of detecting signals released from two detecting regions adjacent to each other in the photodetector.
FIG. 7 is one example of the FES curve showing the relationship between the intensity of the focus error signal thus obtained and the amount of the displacement of the optical disk based on the focal point of the object lens taken as a reference. In FIG. 7, a reference point A represents a case without a focus error. A converging point of the reflected beams diffracted toward the photodetector by the diffracting element varies in its position in front of or behind the photodetector in response to fluctuations of the recording medium. As a result, since a shape of a light spot formed on the photodetector by the reflected beams is reversed, the value of the focus error signal is reversed from positive to negative in accordance with the amount of the displacement of the recording medium.
In this case, as shown in FIG. 7, when the recording medium moves away from the focal point of the object lens to a certain extent, an undesired zero-cross point B appears on the FES curve at a position other than the reference point A. Especially in an optical pickup device using a shaping prism in its optical system, an undesired zero-cross point may appear within a dynamic range of focusing control, depending on what arrangement is selected with respect to the optical system. This is related to the fact that the fluctuation of the recording medium causes the shaping prism to have astigmatism.
Meanwhile, the zero-cross point in the FES curve provides a drive target for the object lens in focus servo control, and if a zero-cross point B appears at a position other than the reference point A as described above, the following problems arise. When focusing control is performed so as to move the object lens from a farther position toward a closer position with respect to the optical disk, for example, in the case of occurrence of an excessive focus error due to an external cause or an initial state of the device, the object lens might be moved to focus on the undesired zero-cross point B. As a result, normal information recording or reproduction might not be performed.